The reality is that state ownership of most industries will have little impact on their finances. Most rail services, for example, will still be loss making and require subsidy. Their plans fail to recognise that its an industry with very high costs, and that won’t change.
Has the Labour party proposed reducing the budget for rail? If not, I wouldn't worry too much about how much they'll save by taking franchises in house as they expire.
Personally, I'd hope nationalisation would lead to better decision making, as TOCs won't be constrained by promises they made a few years ago which everyone accepts they can't keep. They clearly work with the government to manage franchises now, so it's hard to point to any operational decision which would be made differently.
There are other problems which are caused by, or at least exacerbated by, the railway franchise system, such as not having enough on-board staff to meet the railway's needs, but there's nothing specific in the manifesto about that.